wc: diagnose overflow of total counts

wc: diagnose overflow of total counts

* src/wc.c (wc): Use INT_ADD_WRAPV() to detect overflow.
(main): Upon overflow, saturate the total, print a diagnostic,
and set exit status.
* tests/misc/wc-total.sh: Add a test case, which operates
on BTRFS and 64 bit systems at least.
Reported at https://bugs.debian.org/1027100